Metal mirrors have been gaining increased acceptance for a variety of applications because they may be made lightweight, have high stiffness to weight ratio, and have high stability. They are often easy to fabricate and mount and have good thermal conductivity. Also, they are generally not frangible and require relatively short fabrication cycles.
In the past, high quality metal mirrors have been produced by hogging out of a single wrought or hot pressed slab of stock. As the trend for higher quality and lightweighting have progressed, the method of fabricating mirrors has taken an entirely new direction. Numerous thin sections have been entirely gas pressure diffusion bonded for super lightweighting. Mirrors have also been brazed for water cooling passages. In the past, monolithic mirror blanks for more isotropic requirements were cast and plated on the mirror surface to mask the imperfections.
It is known that beryllium may be made into a product which is inherently reflective and used as a mirror. In prior beryllium mirrors, powders have been used and after suiable heating and pressurization, the external surface of the formed articles were suitably machined and polished. Generally, the initial body was a block of material, which had to be machined to the proper shape.
One of the disadvantages of the aforementioned beryllium product is that there are limitations to what can be machined from a solid block. A second disadvantage is that the structure of beryllium is anisotropic. Under these conditions, after the powder is hot pressed, the properties in the vertical direction differ from those in the horizontal direction. This effects the final figure of the mirror so that small temperature excursions cause the mirror to change more in one direction than the other due to this anisotropy. The use of very fine beryllium powder with the proper morphology ameliorates this problem, facilitating equal compaction in all directions.
Beryllium mirrors, when properly constructed, are useful in reflecting long wave and infrared signals. In general, when beryllium mirrors are used in large sophisticated systems, it is important to maintain the figure of the mirror in adverse environments. Such adverse environments, for example, may involve laser or bomb flashes.
It is desirable to have beryllium mirrors relatively lightweight with good thermal properties. These objects are obtainable through the use of hollow substructures. Providing a mirror with hollow areas therein for lightweight and using very fine beryllium powders require special mandrels which are used for the formation of the product but which then must be removed. During the compression of the fine powder, it is important that the spacing between the mandrels and compressed powder be maintained in some predetermined relationship. Consequently the shapes of the mandrels used and their spaced relationship with respect to each other is an important consideration. Spacing of the elements making up the main body of the mandrel is necessary in order to receive the powder for forming the interior structure of the mirror after the mandrel elements are removed.